1. Field of the Invention
The present invention relates generally to battery cell management devices and, more particularly, to a novel technique for bypassing a failed cell of a multi cell battery.
2. Description of the Prior Art
While the present invention is not limited to lithium ion batteries, it has a particular benefit when applied to their use because of their unique operating characteristics. Lithium ion batteries are rapidly becoming the power source of choice for space applications. They exhibit high energy and power both per unit volume and per unit weight in comparison with NiCd, nickel-metal hydride (NiMH), and other rechargeable types.
Because of one of their unique operating characteristics, lithium ion battery cells require careful charge management to ensure that significant over charge and over discharge does not occur. This is for the reason that lithium ion batteries possess an extreme sensitivity to overcharging and overdischarging not found in most other types of batteries. Such charge management may be achieved by limiting the maximum voltage to which the cell is charged. In order to achieve the maximum possible energy stored in the cell while limiting the over charge and over discharge, a device is required that controls the voltage. Also, in many applications, if a cell opens, then the whole battery would be lost. It is desirable to allow the cell to be completely bypassed if it fails in this manner. The ability to monitor temperature and adjust the maximum charge voltage accordingly is also desirable. A feature to allow varying the charge voltage setpoint from outside the device is also desired.
Previous battery cell management devices for nickel hydrogen or nickel cadmium cells typically used diodes or a relay device to short out failed cells. These earlier battery technologies were not as sensitive to over charge as lithium ion cells and consequently did not require devices to precisely control each individual cell voltage.
Other known prior art includes the approach typically used by automotive applications that are presently under development. The approach used is to connect a relay or electronic switch in series with a fixed resistor that is connected around the cell. Some appropriate circuit or computer is used to monitor cell voltage and closes the relay when a maximum voltage limit is reached. The resistor is set to shunt some or all of the current around the cell so that it slows or stops charging. This approach is inexpensive and simple but does not provide optimal charge control and may reduce battery life. In particular, the relay bypass approach may not provide sufficient reliability in a space application where high energy cells are used.
It was in light of the state of the technology as just discussed that the present invention was conceived and has now been reduced to practice.